There is a continuous development of new generations of mobile communications technologies to cope with increasing requirements of higher data rates, improved efficiency and lower costs. High Speed Downlink Packet Access (HSDPA) and High Speed Uplink Packet Access (HSUPA), together referred to as High Speed Packet Access (HSPA), are mobile communication protocols that were developed to cope with higher data rates than original Wideband Code Division Multiple Access (WCDMA) protocols were capable of. The 3rd Generation Partnership Project (3GPP) is a standards-developing organization that is continuing its work of evolving HSPA and creating new standards that allow for even higher data rates and improved functionality.
In a radio access network implementing HSPA, a user equipment (UE) is wirelessly connected to a radio base station (RBS) commonly referred to as a NodeB (NB). A radio base station is a general term for a radio network node capable of transmitting radio signals to a user equipment (UE) and receiving signals transmitted by a user equipment (UE).
3GPP has evaluated the potential benefits of uplink transmit (Tx) diversity in the context of HSUPA. With uplink transmit diversity, UEs that are equipped with two or more transmit antennas are capable of utilizing all of them for uplink transmissions. This is achieved by multiplying a UE output signal with a set of complex pre-coding weights, a so-called pre-coding vector with one pre-coding weight for each physical transmit antenna. The rationale behind uplink transmit diversity is to adapt the pre-coding weights so that user and network performance is maximized. Depending on UE implementation the antenna weights may be associated with different constraints. Within 3GPP two classes of transmit diversity are considered:                Switched antenna transmit diversity, where the UE at any given time-instance transmits from one of its antennas only.        Beamforming where the UE at a given time-instance can transmit from more than one antenna simultaneously. By means of beamforming it is possible to shape an overall antenna beam in the direction of a target receiver.        
While switched antenna transmit diversity is possible for UE implementations with a single power amplifier (PA), the beam forming solutions may require one PA for each transmit antenna.
Switched antenna transmit diversity can be seen as a special case of beamforming where one of the antenna weights is 1 (i.e. switched on) and the antenna weight of any other antenna of the UE is 0 (i.e. switched off).
A fundamental idea behind uplink transmit diversity is to exploit variations in the effective channel to improve user and network performance. The term effective channel here incorporates effects of transmit antenna(s), transmit antenna weights, receiving antenna(s), as well as the wireless channel between transmitting and receiving antennas. Selection of appropriate antenna weights is crucial in order to be able to exploit the variations in the effective channel constructively.
During 2009 and 2010 the 3GPP evaluated the merits of open loop beam forming and open loop antenna switching for uplink transmissions in WCDMA/HSPA. These techniques are based on that UEs equipped with multiple transmit antennas exploit existing feedback e.g. feedback transmitted on the Fractional Dedicated Physical Channel (F-DPCH) or on the E-DCH HARQ Acknowledgement Indicator Channel (E-HICH) to determine a suitable pre-coding vector in an autonomous fashion. The purpose of pre-coding the signals is to “maximize” the signal to interference ratio (SIR) at the receiving NodeB. Since the network is unaware of the applied pre-coding weights the NodeBs will experience a discontinuity in the measured power whenever a change in pre-coding weights occurs. A summary of the 3GPP studies on open loop transmit diversity techniques can be found in 3GPP's technical report TR 25.863, UTRA: Uplink Transmit Diversity for High Speed Packet Access.
Recently there have been proposals for introducing closed loop transmit diversity for WCDMA/HSPA. Closed loop transmit diversity refers to both closed loop beam forming and closed loop antenna switching. At the 3GPP meeting RAN#50 a work item with the purpose of specifying support for closed loop transmit diversity was agreed. Contrary to the open loop techniques where the UE decides pre-coding weights autonomously, closed loop techniques are based on that the network, e.g., the serving NodeB, selects the pre-coding vector with which the signal is multiplied. In order to signal the necessary feedback information from the network to the UE, the NodeB can either rely on one of the existing physical channels, e.g., F-DPCH, or a new feedback channel could be introduced.
Uplink multiple-input-multiple-output (MIMO) transmission is another related technique that has been proposed as a candidate for WCDMA/HSPA in 3GPP standard release 11. A study item on uplink MIMO for WCDMA/HSUPA was started at the 3GPP RAN#50 plenary meeting. For uplink MIMO, different data is transmitted from different virtual antennas in so-called streams, where each virtual antenna corresponds to a different pre-coding vector. Note that closed loop beam forming can be viewed as a special case of uplink MIMO where no data is scheduled on one of two virtual antennas.
MIMO technology is mainly beneficial in situations where the “composite channel” is strong and has high rank. The term composite channel includes the potential effects of transmit antenna(s), PAs, as well as the radio channel between the transmitting and receiving antennas. The rank of the composite channel depends on the number of uncorrelated paths between the transmitter and the receiver. Single-stream transmissions, i.e. beam forming techniques, are generally preferred over MIMO transmissions in situations where the rank of the composite channel is low e.g. where there is a limited amount of multi-path propagation and cross polarized antennas are not used, and/or the path gain between the UE and the NodeB is weak. This results from a combined effect of that the theoretical gains of MIMO transmissions is marginal at low SIR operating point and that inter-stream interference can be avoided in case of single-stream transmissions.
Currently HSUPA does not allow MIMO transmission since only single stream transmissions are allowed. Inner loop power control (ILPC) and outer loop power control (OLPC) are used to control the quality of the uplink transmission. More specifically, the ILPC is located in the NodeB(s) of an active set. The ILPC is used to ensure that a Dedicated Physical Control Channel (DPCCH) pilot quality target Γtarget is maintained. All NodeB(s) in the active set monitor that the received power of the DPCCH pilot fulfills the quality target Γtarget and based on this monitoring these NodeB(s) issue transmit power control (TPC) commands to the UE to raise or lower the transmission power of the DPCCH pilot. Since gain factors for a certain Enhanced Dedicated Channel Transmission Format Combination (E-TFC) are pre-defined power offsets with respect to the DPCCH transmit power, the ILPC implicitly controls the transmit power of all the physical channels. The OLPC is located in the radio network controller (RNC) and is used to control the quality target Γtarget used by the ILPC. Although not specified in the 3GPP standard, the OLPC typically increases the quality target Γtarget if a too high transport block error rate (BLER) is observed.
For uplink MIMO transmissions, the UE needs to transport multiple DPCCH pilots in order to estimate the wireless channel. For instance for 2×2 uplink MIMO, two DPCCHs need to be transmitted by the UE. Data signals associated with different streams and different pilot signals will generally experience different radio link quality. An issue for such settings then becomes power control to ensure reliability and efficiency of UL multiple stream transmissions.